1. Field of the Invention
The present invention relates to the production of hot-rolled elongate hollow products of both round cross-section, such as seamless pipes, and a cross-sectional configuration other than round on the inner and/or outer contours, such as various ducts or finned tubes. More particularly, this invention relates to a working mandrel and a method of rolling elongate hollow pieces in a multi-stand continuous mill on this working mandrel, which is set to travel at a controlled speed.
The invention contemplates a mandrel for use in continuous rolling mills and made in two parts: a working mandrel whereon a piece is reduced in section and a mandrel bar connected to a drive for axially moving the working mandrel, which is to this effect connected with the bar by a suitable means or bears up against the bar. A mandrel bar associated with such a working mandrel may work in tension as well as in compression. The working mandrel forming the subject of the present invention will hereinafter be termed "a working mandrel of the kind described".
2. Description of the Prior Art
Known in the art is a mandrel of the kind described, which is furnished as part of a multi-stand mill for the production of hot-rolled seamless pipes by INNSE-Innocenti Santeustacchio-Italy, developed in cooperation with Dalmine (both associated with FINSIDER). This mandrel has a working mandrel comprising a body having a working surface, the body being of a cross-sectional configuration corresponding to the inner contour of the piece to be rolled. The working mandrel of the prior art mandrel is up to 13 m long.
In mandrels having such a working mandrel the basic problem involved stems from the length of the mandrel, which length causes difficulties in the manufacture of such mandrels, their transportation, and what is more important, in handling the mandrels, especially in the production of large diameter tubing. Moreover, to manufacture such a working mandrel, special equipment is required. In practice, the working mandrel of such mandrels is known to wear out unevenly on its length and the portion of the working mandrel whereon the piece is reduced in section while it passes the first roll stands, shows a greater amount of wear.
A method of rolling pipes on the above-mentioned mill by utilizing the prior art mandrel is as follows. A cooled and suitably lubricated working mandrel having a groove in its trailing end is gripped by a mandrel drive mechanism arranged at the feed side of the mill. A pierced billet is positioned in front of the first roll stand of the continuous mill. The mandrel is introduced into the pierced billet until its nose extends a predetermined length from the leading end of the billet. The billet is then fed into the first roll stand and is rolled on the working mandrel towards the second and subsequent roll stands. The mandrel travels at a controlled speed so that once the pipe has been fully rolled, the mandrel has travelled up to twice the distance between adjacent roll stands. On the delivery side of the continuous mill there are found delivery pinch rolls for stripping the trailing end of the pipe off the mandrel end as the trailing end of the pipe leaves the mill. After the pipe is stripped off, the mandrel is retracted through all the roll stands into the starting position upstream of the mill from which it is kicked off into the mandrel cooling and lubricating line. A new mandrel, previously cooled and lubricated, is positioned in front of the mill and the rolling cycle is repeated.
The practice of the prior art method involves some difficulties. Thus, for example, time-consuming auxiliary operations, which do not coincide, restrain output for, prior to feeding the next shell, a long mandrel is to be retracted into the starting position upstream of the mill, the working mandrel is to be transferred to the mandrel cooling and lubricating line, and a new working mandrel, cooled and lubricated, is to be delivered into position. The retraction of the mandrel over the rotatable rolls results in an uneven wear of its working surface around the perimeter. Furthermore, as the mandrel is retracted the lubricant is transferred therefrom onto the rolls to result in a lower rolling friction.